This invention relates to shaped bodies which contain short fibers or whiskers, moldable ceramic-organic compounds or compositions for use in such bodies, metal or ceramic matrix composites which include such bodies, and methods for producing the shaped bodies and composites.
A need for lightweight, high strength, cost competitive parts by the aircraft, automotive and other industries has led to a demand for improved strength to weight ratio materials such as metal matrix composites. In addition, there is a growing demand by such industries for metal matrix composites having relatively complex shapes which require little or no finish machining.
In general, metal matrix composites have a dispersed inorganic or ceramic reinforcing phase which imparts improved strength and stiffness. For example, some improvement in strength and stiffness can be obtained by incorporating ceramic particles in the matrix. And, a more dramatic improvement can be obtained when short ceramic fibers or whiskers are used as the reinforcing phase.
However, the properties of such composites, reinforced with ceramic fibers or whiskers depend strongly upon the orientation of the fibers or whiskers. For example, composites produced with three-dimensionally random oriented fibers or whiskers have isotropic properties (i.e., directionally independent). Metal matrix composites reinforced with fibers or whiskers having one (planar) or two (unidirectional) degrees of orientation exhibit anisotropic properties (i.e., directionally dependent). Thus, except for simple structures such as stiffeners or struts, metal matrix composites having isotropic properties are more desirable. Also, there is an increasing demand for such composites having complex shapes.
In producing metal matrix composites, it is also highly desirable to obtain uniformly distributed fibers or whiskers in a predetermined concentration. It is also desirable to preform the reinforcing phase (i.e., the ceramic fibers or whiskers) into a shaped structure or preform. This shaped structure or preform can then be infiltrated with molten metal or the like to produce a composite part without significantly damaging the structural integrity of the fibers. In addition it is desirable to obtain a relatively high concentration of fibers in the order of up to 50% or possibly more by volume of a composite to increase the strength, the stiffness or otherwise improve the physical characteristics of the part.
In general, the presently used techniques for fabricating reinforcing phase preforms utilize paper making technology in which the inorganic fibers or whiskers are suspended as a slurry and collected as a mat by vacuum filtration. The densities of such mats are limited and typical contain from 4 to 8% fiber by volume. Such mats have a density gradient in the direction of filtration. In addition the vacuum filtration technique produces a fiber mat wherein the fibers have a two-dimensional planar orientation instead of a more desirable random or three-dimensional dispersion as provided by the present invention. In some cases multiple mats have been superimposed on one another and compressed in an effort to increase the fiber density, however, this approach does not usually increase the density sufficiently and tends to further orient the fibers into a planar array. This approach also tends to break fibers. Furthermore, the shapes produced using the above described preforms are limited to relatively simple shapes which are then subjected to expensive and time consuming machining to produce more complex shapes.
Efforts to produce shaped metal matrix composites having a uniformly distributed three-dimensional random reinforcing phase of ceramic fibers or whiskers, by either conventional powder metallurgy techniques or by mixing the reinforcement phase into the molten metal prior to casting (compocasting) have enjoyed only limited success.
Hood et al., U.S. Pat. No. 4,463,058, discloses a method wherein silicon carbide whiskers are uniformly dispersed and/or distributed in a metal matrix. The method comprises forming a slurry of whiskers and mixing the slurry with an aluminum powder. In this manner, silicon carbide whiskers can be added in amounts of up to 50% by volume of the total finished composite. However, as formed the fibers are oriented relative to the surface of the aluminum particles; and, after forging, the fibers tend to have a planar or parallel dispersion rather than three-dimensional random orientation.
There have been a number of attempts to produce improved preforms and metal matrix composites. For example, Motohiro Yamamoto and Minoru Fukazawa of the Tokai Carbon Company Limited disclose a method for manufacturing fiber oriented preforms as reported in Chemical Abstracts (CA 107(20):181624c) They disperse silicon carbide (SiC) whiskers in molten paraffin wax. And then, the resulting mixture after removing excess wax is cooled to a plastic state and formed by extruding through a nozzle or slit. And then, the extrudate is heated at more than 400.degree. C. for removal of the wax. The manufactured preform consists of oriented fibers which are planar or even parallel. In addition, it is difficult to control and reproduce fiber loading.
A similar approach is taught by Masaharu Oshima, Hiroe Okawa, Katsuhiro Kishi, Toshihiro Manaki and Kenishi Shibata of Nissan Motor Company entitled "Extrusion in Forming Fibrous Preform for Composites," as reported in Chemical Abstracts (CA 108(4):25974g). The Masaharu Oshima abstract discloses extruding a slurry of randomly oriented fibers which tends to produce aligned orientation, and then heating the extruded mass to remove the salt. These silicon carbide whiskers were then dispersed ultrasonically in an aqueous slurry containing an anionic surfactant (Sintrex) and then vacuum formed into a porous disk. The air in the disk was then replaced with molten wax by vacuum impregnation with a molten solution (100.degree. C.) containing paraffin wax, VE722 copolymer, and surfactant Stafoam DO. The impregnated disk was heated at 55.degree. in a mold cavity and then extruded. The manufactured rod was dewaxed at about 200.degree. C., sintered at 800.degree. C. and pressure infiltrated with an aluminum alloy at approximately 800.degree. C. at 800 kg/cm.sup.2. The resulting product showed tensile strength of approximately 57 kg/mm.sup.2. However, this technique also produces preforms having parallel oriented fibers and cannot be used to produce shapes having isotropic properties.
It has now been found that shaped bodies or preforms containing ceramic whiskers and metal matrix composites according to the present invention can be produced with relatively high percentages of uniformly distributed three-dimensionally oriented inorganic fibers or whiskers. It has also been found that such bodies can be produced in relatively complex shapes.
In addition it has been found that novel inorganic fiber-organic compositions according to the present invention can be injection molded without significantly damaging or displacing the three-dimensional orientation of the fibers. And these molded products can be converted into ceramic fiber or whisker preforms according to the present invention.
It has also been found that shaped bodies or preforms for the production of metal or ceramic matrix composites can be produced in accordance with the novel methods disclosed herein. Such methods not only produce preforms and metal matrix composites having consistently reproducible concentrations of uniformly distributed three-dimensionally oriented inorganic fibers or whiskers, but also produce such products economically.